Once that is pressed a launcher will appear and at the top of that is a search bar. Click the search bar, then type in 'Camera'
After typing that in, you should see all the options your system knows related to camera. We only want the first option, simply labelled 'Camera'. Click that and your camera app will open up!
Once the Camera app is open there is a button in the lower left corner where you can switch between your document camera and the built in camera. If it defaults to using the wrong camera that is where you will want to switch that.